The contact process for manufacturing sulfuric acid is highly exothermic in several of its operations. Large amounts of reaction heat are generated in the combustion of a sulfur source, e.g., elemental sulfur, hydrogen sulfide, or a metal sulfide with an excess of oxygen to produce a gas containing sulfur dioxide and oxygen. Further substantial amounts of energy are generated in the catalytic conversion of sulfur dioxide to sulfur trioxide and in the absorption of sulfur trioxide into an aqueous absorption liquid comprising sulfuric acid to afford net production of sulfuric acid.
In the manufacture of sulfuric acid, it has long been conventional to recover the heat of combustion of the sulfur source by passing the combustion gas through a waste heat boiler. Because the combustion gas temperature is typically very high, high pressure steam, e.g., at 40 to 70 bar is generated in the waste heat boiler.
It has also been conventional to recover the heat of oxidation of sulfur dioxide to sulfur trioxide. Typically, combustion gas exiting the waste heat boiler is passed through a converter that comprises several successive conversion stages in each of which the gas stream comprising sulfur dioxide and oxygen is passed over a catalyst for the conversion reaction. For recovery of the heat of oxidation of sulfur dioxide to sulfur trioxide, conversion gas exiting the catalytic converter is typically passed through another waste heat boiler, a steam superheater and/or an economizer for heating boiler feed water for the waste heat boiler. Commonly, the conversion gas exiting the penultimate stage of a multi-stage converter is directed to an interpass absorption tower in which SO3 contained in the gas is absorbed into sulfuric acid, thereby enhancing the driving force for conversion of SO2 to SO3 in the converter stage to which the gas stream is returned from the interpass absorber. The gas must be cooled before entering the interpass absorber, which may be accomplished in an economizer as noted above, and/or by passage through gas to gas heat exchangers wherein a stream returning from the interpass absorber is reheated by transfer of heat from a gas stream exiting the same or another converter stage. The returning gas is reheated to a temperature at which further conversion can occur in the converter stage to which the gas is returned.
In addition to the heat generated by combustion of sulfur and oxidation of sulfur dioxide to sulfur trioxide, a substantial increment of energy is generated by absorption of sulfur trioxide from the conversion gas into a sulfuric acid stream for production of sulfuric acid from SO3. Until the 1980s, this increment of heat, which represents in the neighborhood of 25% of the total heat generated in the contact sulfuric acid process, was wasted to the atmosphere or used only in low level applications such as district heating. Absorption acid coolers constructed of stainless steel were typically operated at a maximum inlet temperature in the neighborhood of 110° C., more typically about 80° C.
U.S. Pat. Nos. 4,576,813 and 4,670,242 describe processes in which an SO3 absorber and absorption acid cooler could be operated to heat a cooling fluid to a temperature of 120° C. or higher by maintaining the strength of the sulfuric acid stream exiting the absorber at a concentration of 98.5% or higher, preferably 99% or higher, and recovering the heat of absorption in a heat exchanger in which the heat transfer surfaces wetted by the acid were constructed of properly selected Fe/Cr alloys.
In the processes described in U.S. Pat. Nos. 4,576,813 and 4,670,242, sulfur is burned in dry air to produce a dry SO2-bearing gas stream containing excess oxygen, and the SO2 stream is passed through a converter to produce a dry SO3-bearing gas stream that is directed to an absorption tower where it is contacted with sulfuric acid for high temperature absorption of the SO3. Absorption acid from the high temperature tower, commonly referred to as a “heat recovery tower,” is circulated through an external shell and tube heat exchanger comprising tubes constructed of an appropriate Fe/Cr alloy. In the heat exchanger, heat is transferred to a heat transfer fluid and recovered in useful form. In commercial implementation of the processes described in U.S. Pat. Nos. 4,576,813 and 4,670,242, heat transferred from the absorption acid generates medium pressure steam that is useful in power generation and/or in co-ordinate process operations.
Typically, the high temperature absorber functions as an interpass tower from which the SO3-depleted SO2 stream is returned to a further converter stage to produce a further SO3 conversion gas stream that is then directed to a final absorption tower. To maximize SO3 recovery and minimize sulfuric acid mist, the final absorption tower is ordinarily operated at relatively modest temperature, for example, about 80° C.
U.S. Pat. No. 5,118,490 describes the recovery of SO3 absorption heat from “wet gas.” The reference discloses options for heating boiler feed water by transfer of heat from heat recovery absorption system (HRS) acid. Boiler feed water for the heat recovery system boiler 15 can be preheated in heat exchanger 19 by HRS acid exiting intermediate pressure boiler 15. Boiler feed water for the sulfur dioxide combustion gas waste heat boiler can be heated with high temperature HRS acid by dividing the acid stream exiting the high temperature absorber between HRS boiler 15 and another heat exchanger 21 for preheating high pressure boiler feed water. The HRS acid preferably leaves the absorber at a temperature greater than 200° C. (392° F.), and steam is preferably generated at ≧450 kPa in HRS boiler 15. In other embodiments, the '490 patent discloses that heat exchangers 15 and 21 can be operated in series, in which case the acid typically flows first through exchanger 21.
U.S. Pat. No. 5,130,112 describes a process in which the energy recovered from the SO3 absorption operation is enhanced by injection of steam into the SO3 conversion gas stream prior to absorption. After steam injection, the conversion gas is preferably passed through an economizer, more preferably a condensing economizer, prior to entry into the absorber. The bulk of the HRS acid exiting HRS boiler 107 is recycled as absorption acid for HRS absorption zone 133, but a fraction 137 is transferred to final absorber 157 as makeup to compensate for product acid withdrawn from the final absorption circuit. The latter fraction passes in series through heat exchangers 139 and 141, in each of which the forward flow acid fraction is further cooled by transfer of heat to boiler feed water. In heat exchanger 141, boiler feed water for both the HRS boiler and the SO2 combustion gas waste heat boiler is preheated to 131° C. (268° F.). Boiler feed water exiting exchanger 141 passes through de-aerator 165 and then is divided between the HRS boiler and the waste heat boiler. The fraction flowing to the HRS boiler passes through heat exchanger 139 where it is heated to 184° C. (363° F.) by transfer of heat from the forward flow HRS acid fraction. The other fraction (at 138° C.; 280° F.) flows through heat exchanger 155 where it is heated by transfer of heat from final stage conversion gas and then through condensing economizer 131 where it is further heated by transfer of heat from third stage conversion gas.
U.S. Pat. No. 4,996,038 describes a process in which dilution water can be added as a vapor to the circulating acid, optionally within the tower. Both U.S. Pat. No. 4,996,038 and U.S. Pat. No. 5,538,707 describe heat recovery in an absorption tower comprising a primary absorption zone into which the SO3 gas stream is initially introduced and a secondary absorption zone, above the primary zone, in which the gas stream is cooled and residual SO3 recovered. Boiler feed water is preheated at relatively low temperatures by transfer of heat from acid circulating through the final absorption tower of an interpass process, and by transfer of heat from acid circulating through the drying tower.
PCT Application WO 2011/139390 describes sulfuric acid manufacturing processes wherein increased fractions of water vapor are introduced into the SO3 conversion gas entering a high temperature absorber, thereby increasing the molar ratio of water vapor to sulfur trioxide to 0.40 or higher. Introduction of water vapor increases the quantity of intermediate pressure steam that may be generated per ton of sulfuric acid produced by transfer of heat from the absorption acid exiting the heat recovery system boiler. This application also discusses the option of extracting further energy from absorption acid exiting the heat recovery system boiler by directing it to one or more auxiliary heat exchanger(s) for heating and/or de-aerating boiler feed water. The boiler feed water is heated to a temperature typically in the range of 180° C. (356° F.), but the major fraction of the absorption heat is extracted in the heat recovery system boiler, thus, limiting the extent to which boiler feed water can be heated by transfer of heat from the absorption acid downstream of the boiler.